ActiveRecord::Transactions::ClassMethods#transaction
uses this exception to distinguish a deliberate rollback from other
exceptional situations. Normally, raising an exception will cause the
transaction method to rollback the database transaction
and pass on the exception. But if you raise an ActiveRecord::Rollback exception, then the
database transaction will be rolled back, without passing on the exception.

For example, you could do this in your controller to rollback a
transaction:

class BooksController < ActionController::Base
def create
Book.transaction do
book = Book.new(params[:book])
book.save!
if today_is_friday?
# The system must fail on Friday so that our support department
# won't be out of job. We silently rollback this transaction
# without telling the user.
raise ActiveRecord::Rollback, "Call tech support!"
end
end
# ActiveRecord::Rollback is the only exception that won't be passed on
# by ActiveRecord::Base.transaction, so this line will still be reached
# even on Friday.
redirect_to root_url
end
end